1. Field of the Invention
This invention relates generally to composite materials and joining methods, and more specifically to ablative composite assemblies and improved joining methods thereof.
2. Description of the Related Art
Current practice necessitates joining different ablative materials for use in high temperature applications. Typically applications where this was necessary occurred in the aerospace industry for missiles or in capsules returning from space. The ablative material was typically a sacrificial layer that protected other portions of a vehicle from catastrophic damage. If the adhesive layer joining the ablative material to other portions of the vehicle deteriorated more rapidly than the ablative material, failure of a portion of the vehicle structure was likely to occur, and often resulted in destructive failure of the entire vehicle.
Currently, ablative materials are joined with various adhesives. Ablative materials are typically cross-linked phenolic resins that may have a number of different fillers. Phenolic resins also vary in terms of polymeric type. Silica fibers that are impregnated with phenolic resins are the basis for making silica phenolic composite assemblies. The properties of silica phenolic composite assemblies depend on a number of factors including cure cycle for the phenolic resin and void content. These variables, amongst others, determine some of the choices that need to be made regarding an appropriate adhesive for joining an ablative composite to another ablative composite or to a substrate that needs to be protected.
Often, for the case of ablative composite assemblies, an adhesive is chosen based on the adhesive's ability to resist a thermal use profile. The choice of adhesive may also depend on convenience factors such as ease of use; need to mix up different ingredients of the adhesive; working time and temperature; curing process for the adhesive; ability to reposition parts of an assembly and maintenance of a desired bond line thickness. Paste adhesives are often used in applications because of some of these factors. In the case of paste adhesives, parts that are to be joined must be coated with the adhesive, assembled and loaded, disassembled for cleaning of extruded adhesive and then reassembled. This assemble-disassemble-clean-reassemble process often introduces air gaps that can weaken the bond joint.
Pneumatic valves for missile applications should be lightweight yet capable of withstanding the environment and effects of hot gasses produced from the missile's engine, which may be a solid rocket motor, which is also known as a gas generator. A gas generator can generate a gas at temperatures exceeding five thousand degrees Fahrenheit (5000° F.). Some valves need not necessarily be capable of withstanding these temperature environments for long periods of time, as the valves may only be required to handle hot gas for short duty cycles.
It has been discovered that hot gas valves, such as exhaust valves, used in applications such as for tactical missiles may use inexpensive lightweight ablative composites for their construction. Often, several pieces of ablative composite sub-assemblies used in such applications must be joined together to create a single pressure-tight assembly. Epoxy and RTV (usually Silicone based Room Temperature Vulcanizing) paste adhesives which have been used for joining ablative composite sub-assemblies in these applications have the disadvantage that they are prone to degrade at extremely high temperatures (greater than 5000° F.) encountered in hot gas rocket exhausts. This degradation usually results in loss of a pressure-tight seal between sub-assemblies that have been joined using paste adhesives.
For the foregoing reasons, it is desirable to provide ablative composite assemblies and improved joining methods thereof.